Potential for Butanol Recovery from Fermentation Broths

Louisiana grows sugarcane on more than 400,000 acres, producing 13 million tons of sugarcane annually. One ton of sugarcane yields as a byproduct, approximately 286 pounds of molasses. This is a production of 1.86 million tons of molasses per year. Molasses contains approximately 50-55 percent simple sugars—sucrose, fructose and glucose. The sugars found in molasses are ideal feedstocks for fermentation to a wide variety of products. The bacterium known as Clostridium beijerinckii optinoii can produce butanol and isopropanol from these sugars. While isopropanol could be sold as a chemical commodity, butanol derived from the fermentation could be used to supplement the nation’s fuel supply.

Butanol production from molasses fermentation can be used to produce acetone, butanol and ethanol. Batch fermentations using 5 percent sugarcane molasses have produced solutions of 1 percent butanol and 0.6 percent isopropanol, while continuous fermentations have produced solutions of 0.7 percent butanol and 0.5 percent isopropanol. Beyond lab-scale experiments, the challenge for commercial production is removing butanol from the fermentation broth in a way that is economical and environmentally friendly. Butanol produced by fermentation can be used as a biofuel because it has an energy content near that of gasoline, and it can be used as a feedstock for other biofuels like biodiesel with blends of up to 16 percent possible. In addition, butanol is compatible with existing engines, storage and transportation infrastructures.

Conventional methods for removing butanol from fermentation broth include distillation and extraction. Distilling the butanol is costly and energy-intensive. Separating butanol from the fermentation broth by extraction requires adding a solvent to separate the butanol from the water in the broth, forming two layers, similar to what happens when oil is mixed with water. This is called liquid-liquid extraction, and the extracting solvent has to be removed from the mixture.

Research at the LSU AgCenter is focused on liquid-liquid separation by increasing the salinity of the solution to recover the butanol. Several salts are present in the fermentation media as nutrients. A saline solution made from these salts and distilled water was added to model fermentation broths to determine its effectiveness at concentrating and separating the butanol from the broth. Model fermentation solutions were made by mixing butanol, isopropanol and distilled water. The butanol to isopropanol ratio in the model solutions was identical to that observed into actual fermentations. Adding the saline solution created a mixture made of two layers. The upper layer is the alcohol-rich, organic layer. The bottom layer is the waterrich, aqueous layer. Results from these experiments showed that higher salinities led to better butanol recovery.

Using molasses to produce butanol would create an additional market for blackstrap molasses. The current price for molasses in Louisiana is $155 per ton, and selling molasses for biofuel production could increase the revenue of the currently $288 million molasses market for Louisiana sugar producers.

April Lovelady is a postdoctoral researcher with the Audubon Sugar Institute, St. Gabriel.

(This article was published in the spring 2015 issue of Louisiana Agriculture Magazine.)

April Lovelady, postdoctoral researcher at the Audubon Sugar Institute, conducts studies on liquid-liquid separation to recover butanol from fermentation broth. Adding a saline solution to the broth creates a mixture of two layers, an alcohol-rich layer on top and a water-rich aqueous layer on the bottom. Photo by A. Denise Attaway